A while back I received a
request from a company who needed a water seepage detector. They wanted a
device that ran on 24vac and detected water using a fully isolated circuit.
They wanted a circuit which switched off 24vac to a load when water was detected.
The circuit below performs this function. By changing the phototransistor’s
position in the detector circuit, the circuit logic can be changed so the 24vac
source can be turned on when water was detected. This might be handy to
activate a pump.

The sensor is
made using two stainless steel probes. The probes would be positioned as needed,
so the water would come in contact with them when the water reached a certain level.
Since the detection circuit is isolated, the two wires leading to the probe could be
located some distance away from the rest of the circuitry.

A 15v supply
is developed using two capacitors connected to the 24vac power source. The
circuit is referenced to the two sources of the AC switching transistor Q1 and Q2.
The AC is first rectified and then filtered. A simple zener diode limits the
DC voltage to 15v. A Schmitt trigger is used to form a 100KHz oscillator
circuit which drives the primary of a small transformer. A ferret bead with a
0.25 inch diameter forms the transformer core.20 turns are used on the
primary and 10 turns on the secondary. The secondary winding is connected to
an optoisolator and to the two water detection probes, through a resistor. The
optoisolator has two infrared LEDs inside and a phototransistor. When the two
probes come in contact with water current flows in both directions through the
isolator’s two LEDs. Light from those LEDs is detected by the internal
phototransistor. The output of the phototransistor is connected to a second
Schmitt trigger, which drives two n-channel FETs. The transistors are wired in
such a way that when turned on they conduct current in both directions. The
transistors can thus switch 24vac power to the load. Using the transistor
shown, the current should not exceed 10 Amps.